The antonyms for the phrase "mark as" are quite varied depending on the context in which it is used. In general, antonyms for "mark as" can be verbs that denote the opposite of marking something as a certain thing, such as "unmark," "erase," or "remove." Alternatively, antonyms for "mark as" can be verbs that represent not identifying something as a particular thing, such as "overlook," "ignore," or "miss." If "mark as" is used in the context of grading or evaluating, antonyms may include "fail," "disapprove," or "dissatisfy." Ultimately, the antonym for "mark as" depends on the intended meaning and context of the phrase.